Summary/Abstract A cell's history determines its fate and physiology, but it has been technically challenging to follow cellular history at a large scale and over extended periods. It has therefore been difficult to answer many important questions in biology and medicine: how are the cells in an organism related through their lineage? When was a cell exposed to a signal? Which neurons were active during a particular behavioral state? Answering these questions is fundamental for understanding the dynamics of multicellular systems. The premise of this proposal is that just as evolutionary history is recorded in genome sequence changes, a cell's history can be recorded by the introduction of changes in the genome. Recent genome editing studies have resulted in the development of a technology called GESTALT that uses the combinatorial and cumulative editing of a genomic barcode to permanently mark cells and through sequencing uncovers their lineage relationships. This and complementary studies have demonstrated the remarkable potential for DNA-mediated recording. The proposed project aims to optimize DNA-mediated recording and combine it with other nascent technologies to (1) record the lineage trees that generate the juvenile brain; (2) record the cellular history of Notch and TGF signaling pathway activation; and (3) record the history of neuronal activity during day/wake and night/sleep. Zebrafish will be used as a model system because of the powerful application of genomic, genetic and imaging approaches and its relevance to other vertebrates, including humans.